I think its a great idea! I used to put about 1500 miles (really 1500, not a typo!) per week on my 240D. I did tweak it a bit with some 3.07 gears and a turbo for highway driving since about a third of my trip was interstate. This knocked about 600 RPM off my cruise RPM, and gave pretty good milage. The rest of the trip was mixed speeds of 45 to 55MPH, so on that run I could get 36 to 38MPG. Pure highway driving at 70 to 75 gives a solid 32 to 34MPG. I do think the 4 speed manual helps milage a bit. I've run both trannys in the car and could never hit the high MPGS with the auto.

The 190D is a more refined car on the highway. They are easier to get good mileage out of and they are quieter. They are also harder to work on because everything is packaged much tighter than on the 240D. If you get a 190D Turbo, the engine management is more complicated too. Personally, I love them both...

Good Luck in your search.
Cheers,
Chris

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1983 Turbocharged 240D
1982 Mazda RX-7
1974 Mazda Rotary Pickup Converted to an EV